The usual mechanism is forced flexion of the distal phalanx, typically when hit with a ball. The extensor tendon may avulse part of the proximal aspect of the distal phalangeal bone (see figure Mallet finger). The avulsed part involves the articular surface.
The finger is usually painful and may be swollen and bruised immediately after the injury. Occasionally, blood collects under the nail (called subungual hematoma).
The affected dorsal interphalangeal (DIP) joint rests in a more flexed position than the other DIP joints and cannot be actively straightened but can easily be passively straightened, usually with minimal pain.
Treatment of Mallet finger is with a dorsal splint that holds the DIP joint in extension for 6 to 8 weeks; during this time, the tip cannot be allowed to flex (eg, when cleaning the finger).
Fractures that involve > 25% of the joint surface or that cause joint subluxation may require surgical fixation.